Preheat oven to 350 degrees.
Heat the cast-iron skillet with 1 tablespoon of olive oil on medium-high.
Add the brats and cook for about 6 minutes while occasionally turning them over until brown.
When cooked remove the brats from the skillet and set aside.
Add the rest of the olive oil into the cast iron skillet heated on medium-high.
Cook the onions for about 1 minute.
Add the peppers, cooking for about 3 minutes, continuing to stir as each ingredient is added. (They should appear slightly soft.)
Add the mushrooms, cooking for 1 minute.
Add the sage, salt, and pepper and stir well.
In a separate skillet saute the apples.
When the apples are done, add them to the cast ion skillet.
Add the brats and broth to the cast iron skillet and stir to combine.
Transfer the cast iron skillet to the preheated oven set at 350 degrees for about 10 minutes. Check to make sure that the sausage is completely cooked and the vegetables are tender.